Prior Art Statement
The following patents are representative of the most relevant prior art known to the Applicant at the time of filing of the application:
______________________________________ U.S. Pat. Nos. ______________________________________ 1,114,665 October 20, 1914 T. B. Allen et al 1,918,317 July 18, 1933 R. C. Benner et al 2,993,814 July 25, 1961 W. Epprecht et al 3,248,346 August 26, 1966 S. Amberg 3,531,678 September 29, 1970 F. J. Schiavone 3,538,231 November 3, 1970 T. F. Newkirk et al ______________________________________
The high temperature properties of metal borides, carbides, nitrides and silicides have been long recognized and utilized in high temperature environments in the form of kiln furniture, bricks, heating elements and the like. Early on in the commercial use of these materials their shortcomings were also recognized. When these materials were used at very high temperatures in the presence of molten metal, oxygen, chlorine or the like, these hostile environments reacted with the material causing deterioration. This occurrance is particularly severe in refractory heating elements such as silicon carbide heating elements.
Many attempts have been made to eliminate this deterioration of the base refractory material by coating the article with a substances or mixture of substance that were more resistant to deterioration than the refractory; these attempts have met with only marginal success.
The Allen et al patent listed above coats the refractory article with a glaze made up, in parts by weight, of 120 parts SiO.sub.2, 18.6 parts Na.sub.2 O, 6.2 parts MgO, 28.2 parts K.sub.2 O, 19.9 parts BaO; this composition is applied to the refractory article and fired at 700.degree. C. to vitrify the glaze. Another glaze utilizes a glaze of a different composition applied to silicon carbide resistors i.e. heating elements. Benner et al teaches the use of a glaze made up of an alkaline earth halide, a mixture of TiO.sub.2 and Al.sub.2 O.sub.3, TiO.sub.2 and CaO, CaF.sub.2 and bone ash, Al.sub.2 O.sub.3 and CaF.sub.2, or a mixture of BaCl.sub.2, CaF.sub.2 and Na.sub.3 AlF.sub.6. These glazes are alleged to increase the life of silicon carbide heating elements 25-50% or more at 1100.degree. C.
Epprecht et al teaches an oxidation resistant coating on carbon or silicon carbide incandescent bars which is first applied as molybdenum silicide and then oxidized at 1300.degree.-1500.degree. C. The bars can then be used at 1500.degree. C. for longer periods than unprotected bars.
Two more protective coating are taught by the Schiavone and Newkirk et al patents. The former coats metal heater wires with a boron nitride coating; the latter protectively coats tungsten, carbon, graphite or molybdenum heating elements with a composition made up of aluminum, tungsten and a metal of the group indium, tin and gallium.
The Amberg reference listed above does not relate to a coating for a refractory article or heating elements. It is relevant for its teaching that the incorporation of silicon oxynitride in the mix used to form a resistance type heating element, enhances the oxidation resistance of the end product. According to Amberg 1% silicon nitride is blended with 94% 8 micron molybdenum disilicide powder and 5% bentonite; the mixture is then formed in the shape of an electrical resistance element and fired in hydrogen at 1200.degree. C. and then air at 1500.degree. C.
The present invention distinguishes over Amberg in that the latter has a small quantity of silicon oxynitride grains dispersed throughout the mass of the heating element, whereas the invention is a refractory article, including a heating element, with a silicon oxynitride (or silicon nitride) coating on the outer surfaces of the article with penetration of the coating to a depth greater than the depth of the external pores.
In 1974 the Applicant developed a silicon oxynitride protective coating. It was applied to silicon carbide heating elements and sold, by the assignee of the present invention, to customers who used them. The results were irratic. Some of the customers reported at 25-50% improvement in the useable life of the rods while others report no improvement or only a very marginal one. The coatings were applied in the form of a slurry of thru-200-mesh (74 microns and finer) type silicon oxynitride. Subsequently, the Applicant investigated this failure and found that the coating had spalled off of the heating elements thus allowing atmospheric attack directly on the silicon carbide. It was observed that the coating had not penetrated beyond the depth of the external or surface pores of the silicon carbide heating element and as a result was poorly attached to the latter.